Fig. 1.

Phylogenetic location of DFI organisms. Since the DFI organisms (Supplementary Material 2) occupy only a few branches, I have adopted a simplified three-“domain” evolutionary tree (Woese et al. 1990) instead of the more detailed trees proposed more recently (Cavalier-Smith 2010). The major DFI clades are circled, and the number of DFI genes in each clade, counted from Supplementary Material 2, are shown in parentheses. The CFBP-containing organisms (blue) are all prokaryotic, while the FCBP-containing organisms (red) are all eukaryotic. The Dinoflagellates are very similar to other microbes with flagella/cilia, such as Apicomplexa and Ciliophora (also see Fig. 4), and therefore, the single dinoflagellate FCBP was not assigned a separate branch to conserve space. Note that the CFBP and FCBP lineages are well separated, making direct gene transfers implausible, thus suggesting their independent evolution. I propose that the DFI-encoding organisms, although they are phylogenetically diverse, constitute a new Operational Taxonomic Unit (OTU)